Photovoltaic devices represent one of the major sources of environmentally clean and renewable energy. They are frequently used to convert optical energy into electrical energy. Typically, a photovoltaic device is made of one semiconducting material with p-doped and n-doped regions. The conversion efficiency of solar power into electricity of this device is limited to a maximum of about 37%, since photon energy in excess of the semiconductor's bandgap is wasted as heat. The commercialization of photovoltaic devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices. The cost of electricity can be significantly reduced by using solar modules constructed from inexpensive thin-film semiconductors such as copper indium di-selenide (CuInSe2 or CIS) or cadmium telluride (CdTe). Both materials have shown great promise, but certain difficulties have to be overcome before their commercialization.
As shown in FIG. 1, the basic form of a CIS, or CdTe, compound semiconductor thin-film solar cell (1) comprises of a multilayer structure superposed on a substrate (2) in the following order, a back electrode (3), a light absorbing layer (4), an interfacial buffer layer (5), a window layer (6) and an upper electrode (7). The substrate is commonly soda-lime glass, metal ribbon or polyimide sheet. The back electrode is commonly a Mo metal film.
The light absorbing layer consists of a thin-film of a CIS p-type Cu-III-VI2 Group chalcopyrite compound semiconductor. e.g copper indium di-selenide (CIS). Partial substitutions of Ga for In (CIGS) and/or S for Se (CIGSS) are common practices used to adjust the bandgap of the absorber material for improved matching to the illumination.
CIS and similar light absorbing layers are commonly formed by various processing methods. These include Physical Vapor Deposition (PVD) processing in which films of the constituent elements are simultaneously or sequentially transferred from a source and deposited onto a substrate. Standard PVD practices and equipment are in use across many industries.
PVD methods include thermal evaporation or sublimation from heated sources. These are appropriate for elemental materials or compounds which readily vaporize as molecular entities. They are less appropriate for multi-component materials of the type discussed here, which may decompose and exhibit preferential transport of subcomponents.
A sub-set of PVD methods are appropriate for single-source, multi-component material deposition, including magnetron sputtering and laser ablation. In these implementations multi-component compounds or physical mixtures of elements, or sub-compounds, are formed into targets. The targets are typically unheated or cooled, and their surface is bombarded with high energy particles, ions or photons with the objective that the surface layer of the target is transported in compositional entirety. In this way complex materials can be deposited. By this means the target composition and molecular structure can be closely replicated in the film. Exceptions occur, most commonly, when targets are made from physical mixtures of elemental or sub-components rather than fully reacted compounds. In such cases the target may be consumed non-uniformly with preferential transport of constituents.
Targets for PVD may be formed by casting, machining or otherwise reforming bulk materials. This includes pressing of powdered materials. Target shapes and sizes vary in different systems and at the end of life the spent target materials are frequently recycled.
In the case of CIS and similar light absorbing layers, evaporation is often used to deposit films of the constituent elements onto the substrate. This may be carried out under a reactive atmosphere of the Chalcogen (Se or Se) or, alternatively, these elements can be subsequently introduced by post processing in reactive atmospheres (e.g, H2Se). Heating of the substrates during deposition and/or post-deposition processing, at temperatures in the range of 500° C. for extended periods, is often employed to promote mixing and reaction of the film components in-situ.
A shortcoming of the conventional multi-source vacuum deposition methods is the difficulty in achieving compositional and structural homogeneity, both in profile and over large areas for device manufacturing. More specifically, device performance may be adversely impacted as a result of such inhomogeneities, including semi-conducting properties, conversion efficiencies, reliability and manufacturing yields. Attempts to remove inhomogeneity through post-deposition processing are imperfect and can generate other detrimental effects. Such post-processing is typically carried out at sub-liquidus temperatures of the thin-film material.
When simultaneous deposition from confocal sources for the constituents is employed, the film composition differs from the designed composition outside the focal region. Such methods are suitable for demonstration, but are more limited in achieving uniform large area depositions as is envisioned for low-cost device manufacturing.
When the deposition is from sequential, or partially overlapping, sources (e.g, strip effusion cells used for in-line processing), the elemental composition of the films vary in profile. Subsequent mixing through thermal inter-diffusion is typically imperfect especially if the processing is constrained by the selection of substrates and other cell components. As an example, in-line deposited GIGS films exhibit graded Ga and In concentrations, consistent with the sequence and overlap of the elemental sources.
In Photovoltaic and other multi-layer devices, reactions and diffusion at film interfaces during deposition or during post-deposition processing may impact performance. For instance it has been shown that Na thermally diffuses from soda-lime substrates into CIS layers. In this instance the effect is found considered beneficial to the performance of the photovoltaic cell. Such effects are a natural consequence of the standard processing and are not independently controlled.
CIGS solar cells have been fabricated at the National Renewable Energy Laboratory (NREL) in Colorado which demonstrate 19.5% conversion efficiency under AM 1.5 illumination. These are small area devices produced by elemental co-evaporation onto soda-lime substrates. Larger area devices, manufactured by vacuum and non-vacuum methods by various entities, on glass, metal ribbon or polymer substrates, more typically demonstrate conversion area efficiencies in the range of 8-12%. It is generally accepted that it is due to shortcomings in the device processing, including the light absorbing layer. In this layer, there can be incomplete compositional non-uniformity, incomplete chemical and/or structural development and/or other defects across area, profile and at the interfaces of the layer.
The aforementioned techniques for manufacturing CIS-based semiconductor thin-films for use in photovoltaic devices have not resulted in cost effective solutions with conversion efficiencies that are sufficient for many practical applications.